An exemplary traveling wave tube (TWT) 100 is shown in FIG. 1. The elements of the TWT 100 are generally coaxially-arranged on along a TWT axis 102. The elements include an electron gun 104, a slow wave structure 106, a beam focusing arrangement 108 which surrounds the slow wave structure 106, a microwave signal input port 110 and a microwave signal output port 112 which are coupled to opposite ends of the slow wave structure 106, and a collector 114. A housing 116 is typically provided to protect the TWT elements.
In operation, the electron gun 104 injects a beam of electrons into the slow wave structure 106. The electron beam has a given power level. The beam focusing arrangement 108 guides the electron beam through the slow wave structure 106. A microwave input signal 118 is inserted at the input port 110 and moves along the slow wave structure 106 to the output port 112. The slow wave structure 106 causes the phase velocity (i.e. the axial velocity of the phase front of the signal) of the microwave signal to approximate the velocity of the electron beam.
As a result, the electrons of the beam are velocity modulated into bunches which interact with the slower microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal causing the microwave signal to be amplified. The amplified signal is coupled from the output port 112 as a microwave output signal 120. After their passage through the slow wave detector 106, the electrons are collected in the collector.
Typically, an individual power supply (not shown) is associated with each TWT and the power supply delivers the necessary bias voltages and currents. Standard protection circuits for TWT's are included in an external power conditioning unit (EPC). The protect ion circuit detects excessive helix current and either rapidly turns off the voltages to the TWT, or shuts down the inverters in a switching power supply to remove the voltages from the TWT. The standard protection circuit is typically a simple voltage-level detector with a short time-constant integrator for monitoring the helix current across a sense state resistor with an integrating operational amplifier, a Schmidt trigger circuit, or both. The detector is designed to detect an arc, fault, or overdrive situation and limit the total energy (in joules) that can be delivered to the TWT in order to avoid damage to the internal electrodes.
Standard protection circuits are typically designed for a TWT driven by a single tone. However, in modern communication systems, it is more common to drive the TWT with multiple tones to increase utilization of the available bandwidth. Prior art protection circuits that use a second sensor, called line sensors, use the second sensor to measure input power to the power supply. The line sensor provides a very slow response and are only occasionally used.
Multiple tones cause large transient power fluctuations in the TWT, which results in complex helix current waveforms compared to single or two-tone operation. When a TWT is driven by multiple tones, the helix current fluctuates with the varying phase of the different tones. Even with a constant average output power, the varying phase of the tones causes large body currents which interfere with the standard protection circuit. The line sensor is not practical for helix applications because it is too slow to protect the helix from excessive helix current.
In order to use a standard protection circuit for a multiple-tone driven application, the sensitivity of the detector circuit must be reduced by raising the peak threshold level, increasing the time constant, or both. The de-sensitization avoids spurious shut down from the protection circuit mistaking multi-tone phase-up with fault situations. However, desensitizing the circuit can lead to excessive energy being delivered to the tube in the event of a fault and excessive average power in the TWT when it is operated with only a single tone.